There is a number of situations in which it is desirable to make high accuracy measurements of a small signal resulting from the cancellation of larger signals applied to a network of passive components. The signal sources have noise that, in general, does not cancel the same way the signal does. This represents one of the main elements limiting the accuracy of this type of measurement. Examples of measurements that use signal cancellation and are subject to the noise limitation are instruments for the precision measurements of passive components, such as LCR bridges, sensor measurement circuits etc. One example of a situation where the application of the general principles is relevant is in the field of metal detectors.
Metal detectors employing simultaneous transmission and reception, known as continuous wave (CW) detectors or very low frequency (VLF) detectors, require that their transmit and receive coils are magnetically decoupled from each other. In the terminology used in this field, it is said that they are in an induction balanced or nulled arrangement. In this way, the weak signals originating from the targets excited by the transmitter are not obscured by the strong transmitted signal. Since the amplifier that is connected to the receive coil has a gain of the order of hundreds, the nulling of the coils (the attenuation of the direct or feed through signal) must be at least of the order of thousands. This requirement adds to the complexity of coil construction and therefore to the total cost. It also precludes or makes much more difficult the integration of CW metal detectors with other types of sensors, such as like ground penetrating radars, and the construction of arrays of metal detectors.
There are types of metal detectors that do not require induction balanced or nulled coils. For example, pulse-induction metal detectors use non-overlapping transmit and receive periods and therefore can use un-nulled coils or even the same coil (named mono-loop) for both transmission and reception. However, these detectors have disadvantages such as higher power consumption, higher cost and significant susceptibility to electromagnetic interference. At the other end of the scale, the very simple beat frequency oscillator (BFO) metal detectors employ a relatively stable internal oscillator and another oscillator constructed around the sensing coil. When the coil is brought in the presence of metal, its inductance changes and so does the frequency of its associated oscillator. The difference between the two frequencies (the beat signal) is amplified and transformed into an audible signal, such that a change in the pitch of the sound indicates the presence of metal. This type of detector is very simple and cheap to make, but is very limited in terms of discrimination and ground rejection.
The advantages of the CW detectors of high sensitivity, lower power consumption and simpler construction would benefit greatly from a coil that does not require nulling. There have been a number of approaches to perform electronic nulling, from the fixed arrangement described in the U.S. Pat. No. 4,030,026 to the adaptive systems described in the U.S. Pat. No. 4,006,407, U.S. Pat. No. 5,691,640 and the U.S. Pat. No. 5,729,143. However, all of these have in common the fact that the electronic nulling is only a fine adjustment applied after the transmit-receive coils are coarsely nulled.
The signal applied to the transmit coil of a metal detector can be either generated by an oscillator that includes the transmitter coil (which therefore tunes itself to the frequency of the resonant circuit) or generated by a separate oscillator and applied to the amplifier that drives the resonant circuit. The second option is also used for non-resonant operation. In all cases, the amplitude of the transmitted signal must be maintained as constant as possible, either through active means, such as feedback control of amplitude, or through careful design of the electronic circuits. If the requirements of amplitude stability are not met, the amplitude fluctuations propagate through the residual coupling between the transmit-receive coils and cause an increase in the detection noise floor. The result is the equivalent of a significant reduction in the sensitivity or the detection depth of the detector.
Most metal detectors that attempt to provide electronic nulling deal with this problem by either deriving the nulling signal from the transmitted signal itself or by using separate signal sources with extremely low intrinsic noise. It should be noted that, because of the parasitic circuit elements, the signal required to balance the receive coil may not be totally in phase or anti-phase with the transmitted signal.
Therefore, the first approach, more suited to analogue processing, requires that signals with adjustable amplitude and phase must be generated from the transmitted signal while maintaining the amplitude fluctuations. Only in this situation can the fluctuations in amplitude of the transmit signals induced in the receive coil be cancelled by the amplitude fluctuations of the nulling signal.
The second approach, more suited to digital processing, has the disadvantage that it requires the sources for both transmit and nulling signals to have a noise amplitude level comparable with that of the receiver amplifier. Because the noise of these two sources is not correlated, their noise adds in root-mean-square terms. Therefore, the noise of these two sources used in an electronic nulling circuit must be lower than that of the source used in the nulled coil set-up, by a factor comparable to the receiver amplifier gain, in order to obtain comparable results.
Based on advances in the digital signal processing and mixed signal components, it is now possible to construct CW metal detectors where the transmitted signals are generated numerically before being converted to electric signals with a digital-to-analogue converter (DAC). At the same time, the received signals are digitised with an analogue-to-digital converter (ADC), with the rest of signal processing performed in software.